Guide to Clear Finishes

From the editors of "This Old House" magazine. Visit www.thisoldhouse.com for home improvement ideas, information, and tips.

Without some kind of finish to protect it from the elements, wood can become as dry and lifeless as day-old toast. Coating wood cabinetry, furniture, or trim with a clear finish, whether you stain it or not, gives it richness and depth while protecting it from knocks, scrapes, and the weather. Use this guide to learn more about choosing and using polyurethane, the toughest of the clear coatings. Then you can be sure you're using just the right finish to bring out the beauty of wood around your house.

What is polyurethane?

It's a kind of super-tough varnish formulated so that its microscopic chains of resin molecules will bond tightly with one another as it dries. The result is a finish that's much more resistant to water, solvents, abrasion, and impacts than traditional varnishes.

Basic choices

OIL, WATER, OR BOTH?

Restrictions on VOC emissions have spurred the development of alternatives to oil-based polys. Consider the different characteristics of each type before you buy.

Oil-based. Turns slightly amber, which warms up a wood's colour. Forms a hard, durable film in a few coats. Dries more slowly than water-based polys, so there's a longer wait between coats. Cleans up with mineral spirits.

Water-based. Looks like milk in the can but dries crystal clear. Good if you're trying to preserve a wood's colour. More watery than oil-based polys, so more coats are needed. Dries quickly. Usually lower in VOCs than oil-based finishes but still requires good ventilation during application. Cleans up with soap and water. Never use ammonia-based cleaners on the cured film.

Water-based oil-modified. Appears cloudy in the can but dries to a tough, amber-colour film, like an oil-based finish. Fast drying and compatible with all woods; cleans up with water. Has the same low VOC levels as water-based polys.

GLOSS, SEMI-GLOSS, OR SATIN?

Choose whichever sheen you like best; there's no difference in durability. Just remember that the glossier the finish, the more it will show any underlying imperfections and any future wear and tear.

INSIDE OR OUT?

Most exterior polys can be used indoors, but interior polys should never be used outdoors; they lack the additives that protect exterior finishes from UV rays.

BRUSH ON, WIPE ON, OR SPRAY?

Every poly has its preferred applicator, typically a brush or cloth. Some polys also come in aerosol spray cans.

Brush-on polys work best on flat surfaces where it's important to build up a durable film. Brushes hold a lot of finish, so you can cover a wide area each time you load them up.

Wipe-on polys are best for contoured surfaces—crown molding and stair balusters—where brushing might create drips. Wipe-ons form thinner coats than brush-ons, so use them when wear isn't a concern.

Spray-on polys come in handy on hard-to-reach surfaces, such as shutter louvers and chair spindles. Aerosol sprays require good technique to avoid drips, and extra prep time to protect surfaces from overspray. Their thin films aren't as tough as the ones you brush on.

Pick the right poly

Each polyurethane is unique, thanks to tweaks in its chemical makeup; choose the one that best matches your particular project.

INTERIOR WOOD FINISHES

GOOD FOR: Cabinets, floors, furniture, and trim such as wainscot, where abrasion resistance and durability are important. This versatile all-around poly provides good protection on a variety of wood surfaces. Available in gloss, semi-gloss, and satin.

GOOD FOR: Doors, cabinets, furniture, and floors. Combines the durability and ambering of an oil with the fast drying time, low VOC content, and easy cleanup of a water-based product. Available in gloss, semi-gloss, and satin.

GOOD FOR: Light-coloured woods and stains where ambering would be undesirable. Blended with acrylic resins, it goes on milky but quickly dries crystal clear. Not as durable as oil-based polys. Available in gloss, semi-gloss, and satin.

GOOD FOR: Carved, embossed, or profiled surfaces where a brush could leave drips. Also useful for hiding superficial scratches in previously finished wood. Its thin film offers moderate protection from abrasion. Available in gloss and satin sheens, and in oil-based and water-based formulations.

GOOD FOR: Furniture, cabinets, trim. Stains and protects bare wood with each coat. Before applying to bare wood, use a wood conditioner to ensure even colour. Smooth between coats with 0000 steel wool. Available in satin and gloss.

APPLY WITH: natural-bristle brush

NUMBER OF COATS: 2

HOURS BETWEEN COATS: 6

VOCs: 450 grams per liter (275 in Southern California)

EXTERIOR WOOD FINISHES

Minwax® Helmsman® Spar Urethane

GOOD FOR: Exterior doors, trim, and furniture. Contains UV absorbers that protect the finish and the wood from the sun's rays. Made with a special blend of oils and resins that allows it to flex as the wood surface expands and contracts. Recoat the finish when it turns dull, typically once a year. Available in gloss, semi-gloss, and satin, and in oil-based and water-based formulas.

Wipe after sanding. Dust interferes with adhesion and leaves unsightly bumps in the finish. Use a cloth dampened with mineral spirits on oil-based finishes; use a water-dampened cloth on water-based finishes. A dry microfiber cloth also works well with both finishes.

DON'T…

Shake the can.

Thin the first coat. Thinning doesn't improve adhesion, and you'll have to apply more coats and use more solvent.

Use tack cloths. Some contain chemicals that prevent good adhesion.

Wax the finish. Wax isn't durable and will interfere with any subsequent coats.

Application tips

When working with any finish…

Choose the right brush.

Use natural bristle for oil-based finishes and synthetic filament (polyester, nylon, or a blend of the two) for water-based and water-based oil-modified finishes. Don't use rollers or foam brushes—they create bubbles.

Dampen the brush with a solvent.

Your brush will be easier to clean, and will go longer between cleanings, if its bristles are dipped first in mineral spirits, if you're using an oil-based poly, or in water, if the finish is water-based. Before using the brush, rapidly roll the handle between your hands inside a cardboard box to eliminate excess.

Pour some finish into a clean container.

Working from a separate container prevents the dust and other particles picked up by the brush from contaminating the finish in the can.

Tap, don't wipe.

After dipping about one-third of the brush into the finish, gently slap it against the inside of the container. Scraping the bristles over the edge of a container leaves the brush too dry to apply a coat of the right thickness.

Always brush or wipe with the grain.

It's the best way to work the finish into the wood pores and ensure an even appearance.

When working with water-based finishes…

Dampen the wood first.

Wipe the wood with a damp cloth to raise the fibers; after the surface dries, knock them back with 220-grit sandpaper.

Apply and move on.

Too much brushing can leave these fast-drying polys rough and ragged.

Don't smooth with steel wool.

It leaves behind tiny bits of steel that you can't see—until they rust. Use sandpaper instead.